Letter
Dimension Minimum Maximum
Height 3-1/2 inches (8.890 cm) 6-1/8 inches (15.557 cm)
Length 5 inches (12.700 cm) 11-1/2 inches (29.210 cm)
Thickness 0.007 inch (0.018 cm) 1/4 inch (0.635 cm)

Any item smaller than the minimum dimensions is not mailable.
For items larger in length, width, or thickness, see large envelopes or packages.
For envelopes, length is the dimension parallel to the address.

For letter-sized items over 3.5 ounces (0.099 kg), large envelope prices will be applied.

Letters that meet one or more of the nonmachinable characteristics below are subject to the nonmachinable surcharge.

Square Envelope

The minimum size for a square envelope is 5 inches x 5 inches (12.700 cm x 12.700 cm).